1. Field of the Invention
The present invention relates to chemical-mechanical polishing (CMP) apparatus and methods primarily for use in processing semiconductor substrates.
2. Background of the Art
In certain technologies, such as integrated circuit fabrication, optical device manufacture and the like, it is often crucial that the workpiece from which the device is to be formed have a substantially planar surface.
One process for providing such a planar surface is to scour the surface with a conformal polishing pad having a fine abrasive thereon, commonly called "mechanical polishing". When a chemical etchant is used in combination with the mechanical abrasive material, the combined action is termed chemical mechanical polishing (CMP). The CMP technique is common for the manufacture of semiconductor wafers used for the fabrication of integrated circuit die.
One recurring problem with CMP processing is a tendency to over-polish the edge of the wafer. This problem is due to the normal stress across the wafer surface not being uniform because the polishing pad deforms at the wafer edge under the polishing force. Such "edge effect" can result in reduced yield of devices from the wafer. This edge effect is described more fully in U.S. Pat. Nos. 5,584,751 and 5,795,215, which patents are incorporated herein by reference.
CMP apparatus is in widespread use in the semiconductor manufacturing industry. Characteristic of earlier CMP equipment is the apparatus 10 shown in simplified form by way of FIG. 1. Here, a circular platen 12 with a soft, compliant polishing pad 14 affixed to the top surface of the platen 12 is rotated by means of a motor (not shown). A wafer carrier 16, holding a semiconductor wafer 18 in place juxtaposed against the pad 14. The wafer 18 is typically held in place either by a carrier film (not shown) one side of which is adhered to the bottom of the carrier 16 and the other side of which adheres to the top of the wafer 18 by suction means; a vacuum; or by means of an adhesive or wax placed between the wafer and the carrier 16. A chemical mechanical polishing slurry 20 is introduced onto the central surface area of the rotating pad 14 from a slurry reservoir by means of a slurry delivery tube 22 and is distributed over the pad 14 by centrifugal force. The wafer carrier 16 is also typically rotated about its axis in the same direction as the rotation of the platen 12.
It was found that when using this early design, there was a deformity of the polishing pad which caused a substantially higher rate of polishing around the edge of the wafer and also, sometimes a ring of lesser polishing rate than the center of the wafer adjacent to high polishing rate outer ring. This problem was alleviated in part, by modifying the wafer carrier 16 configuration so as to include a fixed retaining ring 23 and a moveable retainer ring 24. The fixed retainer ring 23 abuts the periphery of the carrier 16 and extends below the top edge of the wafer 18 so as to prevent the wafer 18 from slipping out from under the carrier during polishing. The moveable retaining ring 24 (see FIG. 2) is spaced from the periphery of the carrier 16 and/or fixed retaining ring 23 so as to form a gap 26 between the moveable retaining ring 24 and the outer wall of the wafer holder. The term wafer holder is defined as the carrier alone when no fixed retaining ring is present or the carrier with the fixed retaining ring there-around when one is present. The pressure applied to the moveable retaining ring 24 can be adjusted independently of the pressure applied to the carrier 16. A more detailed description of a retaining ring-carrier assembly taught in the prior art can be found with reference to the two aforementioned issued patents. However, while this design reduced the extent of the edge effect problem, the problem still exists, but to a lesser degree. It has been found that the edge effect is minimized as the width of the gap 26 is reduced. However, the gap cannat be made too narrow since the moveable ring 24 must not rub against the wafer holder to insure against particulate or mechanical binding of the ring 24. Further, widening of the gap 26 allows the polishing pad 14 to deform inside it, again increasing the edge effect.